Currently, nitride semiconductor laser heterostructures include a plurality of layers that include aluminum alloys. The design of the alloy compositions requires weighing many device performance trade-offs. For some devices, the trade-offs are so severe that all design choices lead to poor overall device performance.
In selecting the level of aluminum composition in the n-cladding and p-cladding layers of UV laser diodes, a low level of aluminum composition is desired in the n-cladding layer and a high level of aluminum content is desired in the p-cladding layer from a modal profile point of view. Such an aluminum alloy profile pushes the optical mode toward the n-side of the device, where the n-type dopant is less optically absorbing, thus enabling the laser diode to realize the low material loss required to operate.
However, such an aluminum alloy profile would lead to very poor material quality because of the large lattice mismatch between the substrate and the active region. Consequently, the crystal defect density will be so high that the laser operation cannot be achieved despite the low material loss. Also, a high level of aluminum content in the alloy composition of the p-cladding layer would lead to a very high device electrical resistance because the activation energy of p-dopants increases rapidly with increasing aluminum alloy composition. This effect can prevent the device from achieving laser operation due to resistive heating.